Many electronic devices, such as portable consumer devices and display devices, contain multiple independent circuits. For example, a smart phone may contain a circuit for driving the display and a separate circuit to receive and interpret a user's touch inputs. Such circuits usually need to be connected to other circuits and components within the device. Conventional wire-based connections are generally not used in modern devices, particularly in portable consumer devices, as wires take up more room within the device and make devices thicker, larger and heavier.
Instead, anisotropic conductive films (ACF) are often used in such devices to electrically connect circuits to other circuits, to the device's display panel and to the substrate. ACF bonding has been used to form electrical connections between flexible printed circuits (FPCs) and the glass-based substrates used in many electronic devices (flex-to-glass). As the consumer electronics industry moves toward using flexible plastic substrates in devices, ACF bonding has also been used to form connections between flexible substrates and PCBs (flex-to-board) as well as between flexible substrates and flexible circuit boards or other flexible substrates (flex-to-flex). ACF is often used to form such electrical connections because the bonding method is reliable, low cost and suitable for high-density connections, particular for fine interconnection pitch applications where the pitch is ≤100 μm.
Background information on ACF bonding can be found in: Ge et al, “Evaluation of Interposers in the ACF Bonding Process”, Electronic Components and Technology Conference, Proceedings. 56th, 2006, p. 1499 to 1503; and Brackell, “Achieving Interconnection with Pulse-Heated Bonding”, Circuits Assembly, April 2000, p. 46 to 50.
Although ACF is widely used, in cases where the interconnection pitch (i.e. the distance between neighbouring electrodes/electrical contacts) is fine (for example ≤100 μm), it is crucial that the upper and lower objects being bonded are aligned and remain aligned during the bonding process. The present applicant has recognised the need to improve the ACF bonding method to reduce the misalignment of interconnections, particularly for fine interconnection pitches.